To the rolling elements for bearings (roller, needle, ball, etc.) used in the fields of various industrial machines and automobiles, etc., high repeated stress is applied in the radial direction. Accordingly, the rolling elements for bearings are required to have excellent rolling contact fatigue properties.
It is known that rolling contact fatigue properties are decreased when a non-metallic inclusion is present in steel. Traditionally, it has been attempted to reduce the content of oxygen in steel as much as possible by steel processes. However, the demands for rolling contact fatigue properties are becoming stricter year by year in response to the high performance and weight saving in industrial machines, etc. Bearing steel material is required to have better rolling contact fatigue properties in order to further improve the durability of bearing parts.
Until now, various techniques for improving rolling contact fatigue properties have been presented. For example, Patent Literature 1 discloses steel material that has excellent wire drawability and rolling contact fatigue properties by properly adjusting the ranges of the contents of elements, such as C, Si, Mn, and Al, and by specifying the number of oxide-inclusions in accordance with the chemical compositions thereof.
However, this technique is used to convert the structure of the steel material into fine pearlite, not into a structure in which spheroidal carbides are dispersed, and hence the rolling contact fatigue properties and wear resistance are insufficient.
Patent Literature 2 discloses bearing steel material that has: a chemical composition which comprises 0.6 to 1.2% of C, 0.1 to 0.8% of Si, 0.1 to 1.5% of Mn, up to 0.03% of P, up to 0.010% of S, 0.5 to 2.0% of Cr, up to 0.005% of Al, up to 0.0005% of Ca, and up to 0.0020% of O, with the balance being Fe and unavoidable impurities; an average chemical composition of non-metallic oxide-inclusions which comprises 10 to 60% of CaO, up to 20% of Al2O3, up to 50% of MnO, and up to 15% of MgO, with the balance being SiO2 and unavoidable impurities; and the arithmetic mean value of the maximum thickness of each of oxides and sulfides, which are present in an area of 100 mm2 in each of ten locations in the longitudinal direction of the longitudinal section of the steel material, are 8.5 μm or less, respectively.
According to this technique, the rolling contact fatigue properties of a member, to which a load acting in the thrust direction is applied, are improved by the inclusions extending and accordingly the thickness being reduced; however, when a load is applied in the radial direction, as in a rolling element, such as roller, needle, ball, or the like, it cannot be said that the rolling contact fatigue properties are sufficient, and it is expected that early peeling may occur.
On the other hand, Patent Literature 3 discloses bearing steel material that has: a chemical composition which comprises 0.85 to 1.2% of C, 0.1 to 0.5% of Si, 0.05 to 0.6% of Mn, P≦0.03%, S≦0.010%, 1.2 to 1.7% of Cr, Al≦0.005%, Ca≦0.0005%, and O≦0.0020%, with the balance being Fe and unavoidable impurities; an average chemical composition of non-metallic oxide-inclusions which includes 10 to 60% of CaO, Al2O3≦35%, MnO≦35%, and MgO≦15%, with the balance being SiO2 and unavoidable impurities; the arithmetic mean value of the maximum thickness of each of the oxides and sulfides, which are present in an area of 100 mm2 in each of ten locations in the longitudinal direction of the longitudinal section of the steel material, are 8.5 μm or less, respectively; and the average section hardness of the steel material at an R/2 position from the surface of the steel material (where “R” is the radius of the bearing steel material) is 290 or less in Vickers hardness.
Also, in this technique, the rolling contact fatigue properties of a member, to which a load acting in the thrust direction is applied, are improved by the inclusions extending and accordingly the thickness being reduced; however, when a load is applied in the radial direction, as in a rolling element, such as roller, needle, ball, or the like, it cannot be said that the rolling contact fatigue properties are sufficient, and it is expected that early peeling may occur.